Reliability assessment and verification of safety instrumented systems with the application of LOPA and FTA in the isomerisation unit of the Isfahan Oil Refinery

The reliability and effectiveness of safety systems in the isomerisation unit of Isfahan refinery complex were evaluated. The methodology consisted of two parts: 1) identification of hazardous scenarios, protection layers, and safety integrity level (SIL); 2) verification of SIL of the system using fault tree analysis (FTA). Out of the 22 identified hazardous scenarios, four scenarios had SIL 1, 17 scenarios had SIL 2, and 1 scenario had SIL 3. The scenario related to stop the outflow of fluid from the pumps with the consequent increase in reaction intensity, rise in temperature, and ultimately the reactor explosion had the most critical points of the production process and SIL 3. In all the scenarios, SIL of the system was in accordance with the required SIL (Target SIL). The results indicated that the intrinsic safe design and use of up-to-date equipment with high reliability led to an acceptable level of risk.